In fact, pdfjam is internally built on the pdfpages package as well (it generates a temporary LaTeX document similar to the one suggested by hsigrist in his answer an then invokes pdflatex on it.) However, using pdfbook directly is more convenient (+1).
– DanielNov 23 '11 at 21:15

I suggest you use booklet. If you use pdfpages you have to compile twice and know the location of the first file. If you already have a pdf that you do not want to edit anymore, you can use any command line tool like the aforementioned pdfbook which is much easier than running LaTeX again.

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